The standard-bearer of the governing FMLN got 1.49 million votes, or 50.11 percent of the total, compared with 1.48 million - 49.89 percent - for Quijano.

ARENA accepts the court ruling "despite the inequalities and unfair conditions" of the electoral campaign, the party said in a statement, vowing to mount "with more vehemence, a democratic, serious, intelligent, honest opposition that is loyal to national aspirations."

Sanchez Ceren and running mate Oscar Ortiz "will be legally sworn-in as president and vice president, but their legitimacy remains in doubt," ARENA chairman Jorge Velado said in an appearance on Telecorporacion Salvadoreña television.

ARENA, which governed El Salvador from 1989-2009, claimed victory a few hours after the polls closed on March 9.

Sanchez Ceren, who was a guerrilla fighter during El Salvador's 1980-1992 civil war, will take office June 1 for a five-year term. EFE